The present invention relates to a hip prosthesis and a method for implanting the same in a patient. In particular, the present invention relates to a prosthesis and method of implanting the same which depends upon two types of fixation.
Hip prosthesis are known in the art. One of the most common types of fixation techniques used for hip prosthesis is fixation by cementation by typically using polymethlymethacrylate cement. Using a surgical device called a broach, a canal is constructed in the femoral shaft of a patient which will accommodate the femoral stem of the prosthesis. However, before inserting the prosthesis but after broaching the canal, cement is poured into the canal and the stem of the prosthesis is fully inserted therein so that the entire length of the femoral stem of the prosthesis is affixed by cementation.
One of the problems associated with cement fixation of hip prosthesis is eventual failure due to aseptic loosening of the prosthesis in the patient. It has been shown that this occurs in as many as 15-20 percent of cemented hip replacements as early as 10-15 years following implantation. It has been shown that the reason for such failure is probably due to certain movement known as micromotion at the bone-cement interface which leads to the generation of polymethylmethacrylate particles which result in a foreign loosening and the eventual failure of the cemented hip prosthesis. This activity has been the subject of research and such research has been documented in an article entitled "Effects of Polymethylmethacrylate Exposure Upon Macrophages" published in the Journal of Orthopaedic Research Vol. 6, No. 6, 1988, and of which the applicant is a co-author.
In addition, other researchers have shown that improvements in cement techniques such as cementation under pressure leads to improved bone cement interlock resulting in increased longevity for the cemented prostheses.
Another and more recent technique of fixation for a hip prosthesis is through the use of a porous ingrowth surface. With this technique, the prosthesis is porous coated to allow bone ingrowth. This technique was developed in an attempt to construct a prosthesis which would last longer than the cemented designs.
At the present time, it is too soon to tell whether a porous ingrowth prosthesis will last longer than the cemented prosthesis. However, it has been shown that poor bone stock or any factor which impairs initial prosthesis fixation will lead to early micromotion at the porous/bone interface. This motion results in fibrous tissue ingrowth, rather than bone ingrowth. This lack of bone ingrowth has been implicated as a likely mode of failure of this prosthesis. Bone ingrowth into porous coated prosthesis occurs maximally during the critical period of the first six weeks of implantation. Accordingly, it would be advantageous to eliminate early micromotion so that a situation would exist allowing the maximum amount of bone ingrowth and resulting in the maximum longevity possible for the porous design. It would also be advantageous to use an additional mode of fixation in association with the porous design, then the design could be used in situations with poor bone stock.